This invention relates to an emission control system for use with a gaseous fuel powered internal combustion engine. More particularly, it relates to a system for producing a source of hydrogen for supplementing the fuel for the engine and for producing a source of ammonia for use in a selective catalytic reduction reactor used to treat the exhaust gas stream, thereby producing an exhaust gas with near-zero NOx emissions.
Hydrogen is known to burn cleaner in internal combustion engines than more traditional fuels such as gasoline and diesel. However, hydrogen is difficult to store, and convenient sources of hydrogen are not readily available. Light hydrocarbons such as methane, propane and butane, and mixtures of light hydrocarbons such as natural gas are more readily available than hydrogen and easier to store than hydrogen. While such fuels tend to burn cleaner than gasoline or diesel, such gases are not as clean burning as hydrogen. Current production engines can often use such alternative fuels without any substantial engine modification, and when operating under lean-burn conditions, such fuels can result in low emission levels that are below current legal standards.
According to U.S. Pat. Nos. 5,660,602; 5,666,923 and 5,787,864 which are incorporated by reference, a clean burning alternative gaseous fuel is disclosed for use in internal combustion engines. Such a fuel includes approximately 21 to 50% hydrogen and the balance natural gas.
According to U.S. Pat. No. 6,397,790 and pending application Ser. No. 09/541,541 which are incorporated by reference, apparatus and methods can be used to produce hydrogen from gaseous fuels for combustion in internal combustion engines. According to these U.S. Pat. and U.S. Pat. No. 6,405,720 which is also incorporated by reference, emissions can further be reduced by using high levels of exhaust gas recirculation in internal combustion.
According to the present invention, a method and apparatus for reducing the emissions from a lean burn internal combustion engine are set forth. The internal combustion engine is generally of the type that burns a mixture of fuel and air, producing a hot exhaust gas stream containing steam, nitrogen, NOx, and oxygen. The method includes a reaction step for reacting a first portion of the hot exhaust gas stream with a source of fuel gas in a reactor to produce an intermediate exhaust stream containing hydrogen. At least a portion of the hydrogen is further reacted with nitrogen to form ammonia. The resulting intermediate exhaust gas stream which now contains ammonia and hydrogen is cooled, condensing ammonia-saturated water. The remaining gaseous components consisting of hydrogen, ammonia, nitrogen, carbon dioxide, and unreacted fuel are then recycled to the internal combustion engine to either supplement the fuel to the internal combustion engine, or if a sufficient amount of combustibles are present, the recycled gas is the exclusive source of fuel for the internal combustion engine.
The condensed ammonia-saturated water is vaporized through heat exchange with the intermediate exhaust stream leaving the reactor. This vapor is then mixed with a second portion of the exhaust gas stream where it is reacted in a selective catalytic reduction reactor to produce a treated exhaust stream substantially free of NOx. Optionally, the treated exhaust gas stream may be further treated by known oxidation reactions to remove any remaining carbon monoxide. The source of oxygen for such oxidation reactions is the hot exhaust gas stream when the engine is operated under lean burn conditions.